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Python | Reverse an array upto a given position

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  • Difficulty Level : Medium
  • Last Updated : 21 Nov, 2018

Given an array arr[] and a position in array, k. Write a function name reverse (a[], k) such that it reverses subarray arr[0..k-1]. Extra space used should be O(1) and time complexity should be O(k).

Examples:

Input: arr[] = {1, 2, 3, 4, 5, 6}
       k = 4

Output:  arr[] = {4, 3, 2, 1, 5, 6} 

This problem has existing solution please refer Reverse an array upto a given position link. We will solve this problem quickly in Python.




# Program to Reverse an array 
# upto a given position 
  
def reverseArrayUptoK(input, k):
  
    # reverse list starting from k-1 position 
    # and split remaining list after k
    # concat both parts and print
    # input[k-1::-1] --> generate list starting
    # from k-1 position element till first 
    # element in reverse order
    print (input[k-1::-1] + input[k:])
  
# Driver program
if __name__ == "__main__":
    input = [1, 2, 3, 4, 5, 6]
    k = 4
    reverseArrayUptoK(input, k)

Output:

[4, 3, 2, 1, 5, 6]
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